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Rosca D. – Bănică L. – ActiveX - An Internet Strategy for Applications DevelopmentAutomation Servers can be in-process (executing in the process space of the Controller),local (executing in its own process space), or remote (executing in a process space on anothermachine). The specific implementation of the server will, in some cases, define how and wherethe server will execute, but that is not guaranteed. A DLL can execute as either in-process, localor remote; an EXE can execute only locally or remotely.Automation Controllers are those applications that can use and manipulate AutomationServers. A good example of an Automation Controller is VB. An Automation Controller can beany type of application, DLL or EXE, and can access the Automation Server either in-process,locally, or remotely. Typically, the registry entries and the implementation of the AutomationServer indicate which process space the server will execute in relation to the Controller.ActiveX Controls (formerly known as OLE control) has a broader definition. It refers to anyCOM objects. For instance, the following objects are all considered an ActiveX control.- Objects that expose a custom interface and the IUnknown interface- OLE automation servers that expose the IDispatch/Dual interfaces- Existing OLE controls (OCX)- OLE objects that make use of monikers- Java Applet with the support of COMActiveX Control used inside scripting languages make this binary reusable componentsreused in the Internet world. Almost any type of media wrapped into an ActiveX control can beseamlessly integrated into your Web page. Sound, video, animation, or even credit-cardapprovals controls can be used within your Web page.4 – COM: The Fundamental "Object Model" for ActiveX and OLECOM (Component Object Model) is the technical cornerstone for the ActiveX technology; itdefines how objects expose themselves for use within other objects and how objects cancommunicate between processes and across a network. You can easily integrate COM objects foruse in many languages, such as Java, Basic, and C++. COM objects are reusable binarycomponents.The following concepts are fundamental to COM:- Interface: The mechanism through which an object exposes itself.- IUnknown Interface: The interface on which all others are based. It implements the referencecountingand interface-querying mechanisms required for COM objects.- © 2007 p. 118
Rosca D. – Bănică L. – ActiveX - An Internet Strategy for Applications Development- Reference Counting: The technique by which an object keeps track of its reference instancecount. The instance of the object class should be deleted when there is no reference to thisinstance.- QueryInterface Method: It is called with the Interface ID (IID) to which the caller wants apointer. Can be generated by Guidgen.exe by choosing DEFINE_GUID(...) format.QueryInterface enables navigation to other interfaces exposed by the object.- IClassFactory Interface: This interface must be implemented for every object class. Itprovides functionality to create an instance of the object class with CLSID and locks the objectserver in memory to allow creation of objects more quickly.- Marshaling: The mechanism that enables objects to be used across process and networkboundaries, allowing interface parameters for location independence by packing and sendingthem across the process boundary. Developers have to create proxy/stub dll for the custominterfaces if exist. The custom interface has to be registered in the system registry.- Aggregation: COM object supports an interface by including another object that supports thatinterface. The containing object creates the contained object as part of its own creation. The resultis that the containing object exports the interface for the contained object by not implementingthat interface.- Multiple Inheritance: A derived class may inherit from multiple interfaces.5 – ActiveX Object ModelThere are two primary pieces to the ActiveX Object Model: the Microsoft HyperText MarkupLanguage (HTML), Viewer component (MSHTML.dll) object and the Web Browser Control(shdocvw.dll). Both are in-process (DLL-based) COM objects.classes.All interfaces defined in the ActiveX Object Model are "dual" interfaces. A "dual" interfacemeans that the objects inherit from IDispatch and IUnknown. They can be used by clientapplication at "early-bind" via Vtable and at "late bind" via OLE automation controller by usingIDispatch::GetIdsOfNames and IDispatch::Invoke.vtable).MSHTML is the HTML viewer part of Microsoft Internet Explorer 3.0. It is an in-processCOM server and a Document Object. It can be hosted in OLE Document Object containers.MSHTML implements the OLE Automation object model described in the HTML ScriptingObject Model. With this object model, you can develop rich multimedia HTML content.VBScript running inline in the HTML and Visual Basic 4.0 running external to the HTML canuse the object model.- © 2007 p. 119
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